All of the following statements regarding glomerular filtration rate (GFR) and its determinants are correct EXCEPT:

Explanation

## Analysis of GFR Determinants ### Correct Statements (Options 0, 1, 2) **Option 0 — Kf and GFR relationship:** **Key Point:** The ultrafiltration coefficient (Kf) = permeability × surface area of the glomerular filtration barrier. GFR is directly proportional to Kf; any increase in Kf (e.g., glomerulonephritis with capillary dilation) increases GFR, and vice versa. ✓ Correct. **Option 1 — Afferent arteriolar resistance:** **Key Point:** Increased afferent arteriolar resistance reduces glomerular capillary hydrostatic pressure (P~GC~), thereby decreasing the net filtration pressure and GFR. This is a key autoregulatory mechanism. ✓ Correct. **Option 2 — Autoregulation:** **High-Yield:** GFR remains constant (autoregulated) between MAP 60–160 mmHg via: - **Myogenic mechanism:** Increased pressure → afferent arteriole constriction - **Tubuloglomerular feedback:** Increased NaCl delivery to macula densa → juxtaglomerular cell signalling → afferent arteriole constriction ✓ Correct. ### Incorrect Statement (Option 3) — **THE ANSWER** **Warning:** Increased plasma protein concentration (↑ colloid osmotic pressure, π) **OPPOSES** filtration, not favours it. Higher π pulls fluid back into the capillary, **DECREASING** the net filtration pressure and thus **DECREASING** GFR. The net filtration pressure is: $$P_{net} = P_{GC} - P_{BS} - \pi$$ Where: - P~GC~ = glomerular capillary hydrostatic pressure (favours filtration) - P~BS~ = Bowman's space hydrostatic pressure (opposes filtration) - π = colloid osmotic pressure (opposes filtration) Increasing π makes the equation more negative, reducing P~net~ and GFR. ✗ **Incorrect.** ### Clinical Pearl **Dehydration** increases plasma protein concentration → increases π → decreases GFR. Conversely, **hypervolemia** dilutes plasma proteins → decreases π → increases GFR (transiently, until autoregulation adjusts afferent resistance). --- ## Summary Table | Factor | Effect on GFR | Mechanism | | --- | --- | --- | | ↑ Kf | ↑ GFR | More permeable/larger surface area | | ↑ Afferent resistance | ↓ GFR | ↓ P~GC~ | | ↑ Efferent resistance | ↑ GFR (initially) | ↑ P~GC~ | | ↑ Plasma protein (π) | ↓ GFR | ↑ Opposing force | | Autoregulation (60–160 mmHg) | Constant GFR | Myogenic + tubuloglomerular feedback | [cite:Guyton and Hall Textbook of Medical Physiology Ch 26]